Not applicable.
The present invention relates to semiconductor heat dissipation and more particularly to a spring clip for securing heat sinks to electronic devices (e.g. TO-218, TO-247, TO-264, TO-3P, TO-3PL, etc.) and a unique inverter/converter power device configuration.
Semiconductor switching devices generate heat which must be dissipated to maintain device integrity. One way to dissipate device heat is to provide heat sinks. A sink typically includes a thermally conductive material attached to a device. To increase dissipation efficiency, most sinks include a plurality of dissipation fins or apertures which increase the amount of sink surface area exposed to the ambient (i.e. increase radiation area). In addition, most devices include a primary heat dissipating surface (i.e. a baseplate) which is securable to the sink to facilitate efficient heat flow.
Some mechanisms for securing a semiconductor switching device to a sink include a simple screw (see U.S. Pat. No. 5,592,021, FIG. 1, Prior Art), a clamp and screw (see U.S. Pat. No. 4,259,685), or a clamp integrally attached to a sink (see U.S. Pat. No. 5,068,764). Unfortunately, while these mechanisms, when properly employed, can prevent device overheating, they have a number of shortcomings.
For example, some of these mechanisms provide uneven pressure on the semiconductor devices causing the primary heat dissipating surface to buckle whereby a portion of the dissipating surface is raised away from the sink. This separation results in a significant reduction in reliability and increases heat transfer resistance, thereby reducing dissipation effectiveness.
In addition, these mechanisms can cause what is referred to as xe2x80x9cvoltage creepxe2x80x9d. Because sinks have to be thermally conductive, most sinks are metallic. When a device is connected directly to a metallic sink, in addition to making thermal contact, the sink and device make electrical contact. When any connected metallic components are at different potentials, the potential xe2x80x9ccreepsxe2x80x9d along the metallic surfaces and can cause unintended and undesirable voltage stresses or electrical shorting in the sink and switching devices, hence the phrase xe2x80x9cvoltage creepxe2x80x9d.
Moreover, device/sink configurations often require a large amount of space. This is particularly true in applications which require a large number of switching devices. One such application is a converter/inverter for changing AC to DC voltage and DC to AC voltage. As well known in the art, at a minimum, six separate switching devices are required to efficiently convert DC to AC voltage and another six devices are required to rectify AC voltage and provide DC voltage (assuming a standard three phase system). Space is often saved by providing the rectifying devices in a single integrated in-line package (SIP). Nevertheless, the SIP, like the DC to AC devices, generates an appreciable amount of heat which must be dissipated. Because dissipation effectiveness typically increases with exposed sink surface area, large and/or separate sinks are often provided for each power device and another for the SIP, resulting in a configuration which requires a substantial amount of space.
One other problem with conventional securing mechanisms, is that a relatively large number of components are required to secure devices to sinks. As evidenced by the art cited above, typical securing mechanisms may include a plurality of mechanical components (e.g. clamps, screws, etc.) for connecting each device to an associated sink. Extra components increase hardware costs and assembly time.
The industry has recognized and attempted to address at least some of the problems identified above. For example, to eliminate or reduce voltage creep, a thermally conductive, electrically insulative and mechanically separating layer of material is often positioned between the primary heat dissipating surface of a semiconductor device and a heat sink. In this manner, heat is dissipated but voltage is blocked.
One solution which addresses many of the problems described above is disclosed in U.S. Pat. No. 5,450,284 which issued on Sep. 12, 1995. The assembly described therein uses a single clamping device bolted to a single heat sink to secure a plurality (i.e. 4) of semiconductor devices to the sink. The devices are separated from the sink by two separate thermally conductive and electrically insulating insulators which eliminate voltage creep.
While this solution reduces the overall mechanical parts count, eliminates voltage creep, provides even pressure on each device thereby eliminating device buckling and reduces overall device/sink space, this solution still has several shortcomings. First, this solution still requires several securing components and most of the components are relatively complex. For example, the clamp requires a separate retaining finger for each device secured to the sink and all components require a number of precisely located apertures. Second, this solution is difficult to assemble (e.g. has many different apertures and elements which must be precisely aligned). Third, this solution requires an elongated, relatively large space to accommodate a plurality of separate switching devices. For example, in order to configure an inverter/converter with this solution, all six required switching devices and the SIP would have to be placed next to each other in a single row on a single sink. While such a configuration might be possible, the sink length required to accommodate so many devices in a single row would render the assembly to large for many applications. In the alternative, two or more separate assemblies including a separate sink for each assembly could be configured according to this solution and the separate assemblies could be positioned in parallel to provide inverter/converter power devices. This, however, would increase the parts count and also the space required to house the power components.
Therefore, it would be advantageous to have an apparatus for inexpensively and easily securing a heat sink to a semiconductor switching device and, in addition, it would be particularly advantageous to have such an apparatus for securing together inverter/converter power components so as to efficiently dissipate heat, eliminate voltage creep and require minimal space.
The present invention includes a resilient spring clip for coupling a semiconductor switching device to a heat sink. The clip is formed of stainless steel and includes a base member and two essentially oppositely facing leg members extending from opposite ends of the base member. The leg members can be forced apart such that the device and sink can be placed therebetween in a predetermined configuration. When the legs resume their original position, the legs press against the sink and device sandwiched therebetween with sufficient force to maintain the sink and device in the predetermined configuration. Thus, a single piece clip can be used to secure a device and a sink together. Preferably the clip is formed of stainless steel.
The invention also includes a xe2x80x9cpower structurexe2x80x9d assembly consisting of an inventive clip, heat sinks and the power devices required to configure an inverter/converter for rectifying AC voltage and converting DC to AC voltage. To this end, the brick includes a clip securing together six semiconductor switching devices, a SIP configured to rectify AC voltage, at least one heat sink and at least one thermally conductive and electrically insulating insulator. Preferably the sink includes first and second sinks, the insulator includes first, second and third insulators and the brick further includes a spacer.
In this case, the first insulator can be sandwiched between a first device pair and a first sink first surface, the second insulator can be sandwiched between a second device pair and a first sink second surface opposite the first sink first surface, the SIP can be placed against a second sink second surface, the third insulator can be sandwiched between a third device pair and a second sink first surface opposite the second sink second surface and the spacer can be sandwiched between the second and third device pairs. The first leg member contacts the second surfaces of the first device pair while the second leg member contacts a SIP second surface opposite the SIP first surface, with sufficient force to maintain all of the components therebetween in the predetermined configuration.
Thus, a primary object of the invention is to provide a simple, inexpensive and easy to assemble mechanism for attaching a semiconductor device to a heat sink. To these ends, the inventive clip includes only three resilient and integrally connected members which can be forced from a rest configuration into a configuration wherein a device and heat sink can be sandwiched therebetween. No screws or bolts are required. No apertures need be formed in the device or sink.
Another object of the invention is to provide a single clip of the above kind which can be used to secure more than one semiconductor device to one or more heat sinks. To this end the inventive clip can be used to sandwich several devices and one or more sinks between the leg members.
Yet another object of the invention is to provide all of the power components required to configure an inverter/converter in a single compact module referred to herein as a xe2x80x9cpower structurexe2x80x9d. To this end, all of the power components and required heat sinks and insulators can be positioned such that the boundaries between adjacent components are perpendicular to pressure provided via the leg members. In this manner, all of the components can be sandwiched between the leg members and held in desired positions.
In one aspect the base member includes anterior and posterior edges, the first leg member includes at least first anterior and first posterior leg members extending from the first end adjacent the anterior and posterior edges, respectively, and the second leg member includes at least second anterior and second posterior leg members extending from the second end adjacent the anterior and posterior edges, respectively.
One other object of the invention is to provide pressure on all of the semiconductor devices in the power structure despite the fact that some of the devices might be positioned next to each other. To this end, the anterior and posterior legs cooperate to independently provide pressure across the power structure. When a section of the power structure components aligned with the anterior legs defines a distance smaller than a section aligned with the posterior legs, the anterior legs contract more than the posterior legs and both sets of legs are functional to maintain brick components therebetween.
In another aspect the leg members extend from a first side of the base member and the base member is pre-bowed such that, prior to positioning the semiconductor and sink components between the leg members, the base member is concave toward the first side. In addition, prior to positioning components between the leg members, the base member is concave to a first degree and, after the components are positioned between the leg members, the base member is concave to a second relatively reduced degree such that the base member is essentially flat.
One other object of the invention is to provide a clip of the above kind wherein, after components are secured together via the clip, the clip itself requires minimal space. To this end, the inventive clip is pre-bowed so that, when stretched to accommodate components therebetween, the base member becomes essentially flat requiring very little additional space.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefor, to the claims herein for interpreting the scope of the invention.